How To Pick The Right Camshaft

When it comes to building a powerful engine, there is a hierarchy to the parts choice from most to least critical. The right set of well-designed cylinder heads with high airflow and good chamber design are far and away the parts that set the limit of the engine’s power potential, but no matter how good the cylinder heads could be, there is one part that determines how well they work on any given engine combination: the camshaft.

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That’s because not a single thing in the engine matters at all if the cam isn’t correct for the parts choice, or even more importantly, what the engine is intended to do. Cam selection will make or break an engine more than any other single part because it’s the conductor of the orchestra that is the valvetrain. Awesome cylinder head airflow numbers mean nothing if the valves aren’t opening at the right time, high enough, long enough, or in the correct relation to one another.

While it may take years of schooling and an engineering degree to fully comprehend all the intricacies, it’s fully possible to have a solid grasp of camshaft fundamentals that will put you in the top 10 percent of hot rodders and make your next cam selection more than just a guesswork finger scroll through part numbers. It all begins with fully understanding the basics, and most importantly, how those basics apply to you and your car on a practical level.

3/15The heads are the part that truly dictate what the potential of the engine’s airflow will be, but they must be paired with a cam with specs that complement their flow numbers and the intended operating range of the engine.

"…not a single thing in the engine matters at all if the cam isn’t correct for the parts choice…"

Before You Start

The very first and most critical thing you have to consider when building any engine or selecting a cam is the thing most often glossed over: What exactly are you going to do with this engine? That sounds like a very basic question that you’d assume most guys have established before contacting a builder or ordering parts, but in his experience, Mike Consolo of QMP Racing Engines says the vast majority haven’t been really honest with themselves. The result is almost always a customer who’s not happy with the result.

4/15There’s a significant difference between building a race engine and building an engine that can be raced occasionally. Make sure you’re realistic about what you’ll be doing with your car.

Being honest with yourself begins with forgetting about the specific horsepower numbers you want, and focusing only on what exactly you’ll be doing with the car during the majority of the time. “The very worst thing you can do is over-cam an engine,” Consolo says. “Lift, duration, and lobe separation angle need to be matched to the true end purpose; getting any one of those wrong can over-cam the engine, which is worse than under-camming.” That’s from both a power and enjoyment perspective. Lots of guys say they want 600-plus horsepower and 10-second timeslips, but unless you truly plan to race a lot, this isn’t the right way to approach the engine build. Don’t build an engine oriented toward high rpm if you aren’t going to be spinning it hard regularly. If you’ll actually be spending the majority of your time behind the wheel street driving and cruising at low rpm and with lots of idling time, but only occasionally autocross or run-what-ya-brung drag racing a handful of times a year, keep that solidly in your mind. Starting from any of those perspectives doesn’t mean you can’t build a powerful engine, but it does dictate the type of cam you should choose. As Crane Cams’ Allen Bechtloff once told us, “The right horsepower and torque at the right place is more important than the peak.” The “right place” correlates with the engine’s intended use, and the vehicle the engine will go in.

The other half of the honesty equation is realistically addressing the car you’re going to drop the engine into. Just like an engine is a system that needs to work in harmony, so is the car. You’d be surprised how sluggish a high-powered but peaky or high-revving engine can feel in a car with a poorly matched trans, converter, and rearend ratio. Conversely, you’d be surprised how much quicker any power level feels when properly paired. For example, there’s no point in having a large, lopey cam if you want to run an automatic with a stock stall converter. Similarly, sticking with a puny freeway-friendly rear gear ratio that won’t allow a large cam to ramp quickly into its intended rpm range will only result in a soggy feeling off idle. Other important points to establish include whether you need enough vacuum for power brakes or other vacuum-operated accessories, what type of exhaust will you be running, how much the car weighs, what altitude you live at, whether the engine is EFI or carb, does it have air conditioning, and what kind of driveability and idle quality you want.

5/15Although a cam can always be installed straight-up per the manufacturer’s specs, sometimes horsepower can be found through minor adjustments to the installed position. In any situation, you should always check your cam event timing (with a degree wheel like this) against the cam card to ensure your cam is delivering the advertised motion.

So the very first thing to do before talking to an engine builder is to create a list answering all of those questions. Then at the top write your realistic expectations of what you’ll be doing with the car 90 percent of the time, and how you would like for it to behave. Underneath that create a full list of the drivetrain, suspension, chassis, wheels, and tires you plan to run. This will help paint an accurate picture for the engine builder.

6/15The Cam Doctor is a valuable tool engine builders use for measuring and verifying all the specs of any camshaft. The resulting specs go far beyond what a degree wheel can offer. Check out the video on YouTube.com/PopularHotRodding where Mike Consolo of QMP Racing Engines explains this amazingly useful machine.

The Cam Comes Last

To get the most out of a cam, choose it last. A common mistake is to assume that the camshaft choice should come first, and then the engine should be built around that. It’s actually the opposite, Consolo says. “I prefer to build a cam around an engine, not the other way around.” That’s because an internal combustion engine is just an explosion-driven air pump and to determine how and when the valves need to open is determined by the engine’s airflow potential.

Overall cylinder head flow is important, but there are two other points to consider: the flow in the rpm range that you’ll be using your engine in, and the intake-to-exhaust flow percentage. The first one easily explained; it doesn’t matter what the heads flow at .800-inch lift if you’re building a street engine that will stay in the sub .600-inch lift range. On the other side, if cylinder head flow stalls at .600-inch lift, it makes no sense to run a cam that makes .800-inch lift. But it does matter what the numbers look like at the max intended lift and below. The intake-to-exhaust flow percentage is the amount of air the exhaust port can flow versus the intake port. Typically, a head with a high percentage can use a cam with more closely matched intake and exhaust lobe figures. Conversely, a head with a poor ratio between the intake and exhaust flow will benefit from more exhaust duration to evacuate the burned gases.

7/15This diagram from COMP Cams is an end-on view of a camshaft and shows what the specs on the cam card are referring to.

"Being honest with yourself begins with forgetting about specific horsepower numbers you want, and focusing only on what exactly you’ll be doing with the car..."

Understanding Cam Cards

The cam card provides the critical specs of the cam. The first section provides general information, such as the part number, grind number, the engine family, and if there are any special instructions. Following that are valve lash adjustment recommendations since the example above is a solid lifter cam. The basic lift and duration specs are next, which are always calculated on the factory rocker ratio for that engine (1.6:1 in this case). Next are the exact timing of the intake and exhaust opening and closing events in reference to piston location in the cylinder Before (B) and After (A) Top Dead Center (TDC) and Bottom Dead Center (BDC). The next section lists the cam’s intake centerline, duration at the industry standard .050 inch, the lobe lift, and the lobe separation angle. These are numbers that can be used to compare other cams listing the same standardized specs.

8/15Rocker arm ratios always affect the actual valve lift and duration since they transfer the pushrod motion. Raising the ratio slightly has the effect of installing a cam with slightly higher numbers. These are 1.7:1 Crane Gold rockers, and will multiply by the tappet lift at the cam by a factor of 1.7.

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Lift

This is the easiest term to understand. Lift refers to how far the valve is raised from its seat in the cylinder in inches. There are two types of lift commonly referred to: gross and lobe. The lobe lift is the actual size of the lobe on the cam measured at the nose of the lobe. The gross lift is the lobe lift multiplied by the rocker ratio to give the effective lift that the valve will see. A cam card will always refer to the engine’s stock ratio, but the gross lift can be altered by changing the ratio of the rockers, i.e. 1.5 to 1.6. So, yes, you can slightly upgrade your cam without actually changing it. If aftermarket rockers with a non-stock ratio are being used, the lobe lift multiplied by the rocker ratio will provide the valve lift number. For example, the COMP Cams XE 268H hydraulic flat-tappet cam for small-block Chevy has a lobe lift of .318 inch on the intake side. The effective lift from this can be altered from .477 inch to .509 inch and .541 inch with 1.5:1, 1.6:1, and 1.7:1-ratio rockers, respectively.

10/15For a little better visualization of the sequence of events, this diagram places the cam lobes and the valve’s events on a timeline of 360 degrees of crankshaft rotation.

Duration

While lift refers to how far the valve is opened, duration tells us how long it is open in degrees of crankshaft rotation. That measurement comes in two forms: advertised duration and duration at .050-inch lift. The advertised duration will always be a larger number since it starts lower on the lobe closer to the base, but the problem is that it is not measured from a standardized point that is always comparable to other cam brands. For example, Crane measures their advertised duration at .004-inch valve lift, while COMP measures at .006-inch lift, making the Crane cam sound like it has a longer duration when, in fact, the lobes could be similar. To address the situation, nearly all camshaft companies provide the duration at .050-inch tappet rise, which allow for a good basis of comparison.

Lobe Separation Angle (LSA)

Also sometimes called the lobe displacement angle (LDA), or lobe spread, the LSA is an expression of how the intake and exhaust lobes are phased with each other. Measured in degrees of cam, the LSA is the distance between the centerline point of peak lift on the intake lobe and the peak lift on the exhaust lobe. So while the lift and duration tell what you need to know about an individual lobe, the LSA tells how those lobes relate to one another on a cylinder and how much overlap is present (the brief window when both the intake and exhaust valves are open). Generally speaking, the vast majority of cams will fall in the 104-116 range with nice idling, a broader torque range, and automatic-trans-friendly cams having a wider angle. Racier, faster revving, rougher-idling cams with more midrange torque typical have a narrow LSA. Even more than lift and duration, the LSA really does create the attitude of the cam.

Installed Centerline Angle (ICA)

The installed centerline is a measurement of the relative position of the cam timing versus the engine’s crankshaft position. This measurement is referenced by the crank degrees from TDC at which the cam’s intake lobe reaches max lift. If a cam is installed straight-up, then its ICA will be equal to the LSA, however, the cam’s ICA can be advanced or retarded relative to the crankshaft timing to dial in the desired performance from the engine. Advancing causes valve events to happen earlier, while retarding delays them. Generally there is additional low-speed torque, vacuum, and idle quality to be found by slightly advancing the cam, but it is highly variable depending upon the cam’s specs and the engine family. This is one place an experienced builder with a degree wheel is invaluable. Beware that if you need to advance or retard a cam more than 2 degrees from what is recommended, you probably have the wrong cam to begin with.

Valve Opening & Closing Events

The lift, duration, and LSA combine to establish the valve opening and closing events. These events are what make an engine function and are expressed in degrees of cam rotation and referenced to piston location in the cylinder Before (B) and After (A) Top Dead Center (TDC) and Bottom Dead Center (BDC). We’ve heard Judson Massengill of the School of Automotive Machinists (SAM) express during bench racing at PHR’s AMSOIL Engine Masters Challenge that the intake closing point is by far the most important of the four valve event points. Considering SAM always has a strong showing, we’d say Massengill has a strong grasp of the concept of valve timing.

Types of Camshafts:
Flat Tappet Vs. Roller

The main difference between flat tappet and roller cam types is how they interact with the lifter. A flat tappet is so named because the lifter surface appears to be a flat plane. It’s actually very slightly convex (and the cam lobe is ground with a corresponding taper), which helps it rotate as the cam lobe slides across it. Failure of the lifter to rotate would actually result in very accelerated cam and lifter wear that would eventually cause failure. Roller cams, on the hand, look like a tiny caster. The wheel rolls across the cam lobe so spinning is not required to reduce wear. Moreover, roller lifters are always keyed in place by the lifter bore or through link bars that attach one to another.

14/15The lobes between flat tappet and hydraulic cams look very different even if the specs are similar. This is due to how the lifter interacts with the lobe of the cam. Rollers require a less dramatic nose profile (left) to create the same duration as the flat tappet.

While there are obvious wear benefits to a roller versus a flat tappet, there are performance gains to be had as well. From a practical standpoint, flat tappets are limited mostly by spring load and rpm. They are still used by many successful racers, but the trade-off is in longevity of the parts. With conservative valvesprings, flat tappets can last many miles, but aggressive profiles and high rpm require much heavier springs and the pressure increases wear dramatically. There is definitely a point at which it is too great for real-world hot rodding applications. Rollers, on the other hand, can tolerate much higher spring loads and rpm thanks to the wheel and the different geometry of the cam lobe. Essentially, you can spin harder with higher lift and duration for longer with a roller cam. That geometry and greater control tends to tame an aggressive cam relative to a comparable flat tappet. Another practical benefit to a roller cam is that is does not require the traditional break-in that a flat does, which eliminates one possible source of early engine failure. The break-in is extremely critical for flat-tappet engines and must be performed properly to ensure a good lifespan. Don’t ever skimp on the lifters either, always go with a reputable brand of flat tappet lifter to ensure its surfaces are sufficiently hard to survive the pressures.

So are there any benefits to flat tappet cams? Yes. First and foremost is cost. Flat tappets are always the cheaper route due to their inherently less complex manufacturing. Flat tappets also offer the opportunity for increased ramp speed on the cam lobe (how fast the valve opening event begins to happen) because the surface area of a flat tappet lifter is greater, which can be beneficial in some applications.

Hydraulic Vs. Solid

From a hot rodder’s perspective, the key trade-off between a hydraulic and solid cam will be rpm and valve lift potential versus increased maintenance and wear. The key difference between these two types of cams is mostly in the lifter design. Hydraulic cams, whether flat tappet or roller, use a lifter that feature an internal mechanism that continuously adjusts to maintain zero valve lash. That’s ideal for creating a very quiet, maintenance-free valvetrain, which is why they are far and away the most common style of lifter found in any street engine from vintage muscle car to new sedan. That very same mechanism is also the limiting factor when rpm begins to exceed 6,500 rpm (or the cam profile becomes extremely aggressive with high lift or velocity); it can lead to instability in the valvetrain and valve float because the hydraulic internals can actually begin to compress when overstressed.

15/15For either flat tappet or roller, hydraulic or solid, it’s critical to make sure the valvesprings are matched to the cam. The rate and the force of the spring needs to be appropriate to control the valve action, but not so high that it creates unnecessary stress or wear.

Solid cams use a lifter than has no such internal adjustment, which gives them an extremely precise transfer of the cam profile to the valvetrain. Their tolerance for high rpm and aggressive profiles means you’ll see them almost exclusively in racing circles and applications where high power and performance are the primary goals. As with hydraulic flat tappets, solid flat tappets are limited in how much spring pressure they handle without severely accelerated wear, but they do have a higher rpm potential. Solid rollers offer the increased spring load capacity of their hydraulic counterparts while offering 7,000-plus rpm stability with ease. The big trade-off with both types of solid lifter cams is that periodic valve lash maintenance is required to keep it within spec, or engine damage can occur. That same direct transfer of the cam profile without any damping also results in increased valvetrain wear, so longevity of parts is diminished versus a hydraulic. Both points are definitely something to keep in mind if you plan on dropping the engine in a daily driver.

One last point to note is that solid and hydraulic cam specs on a cam card are not directly comparable. That’s because the standards are measured at different points on the lobe. For example, COMP Cams’ advertised ratings are usually checked at .008 on a hydraulic, but at .020 on a solid. Even the industry standard .050 measurement isn’t a direct comparison since the requisite lash of the solid roller must be taken into account. That means that a solid and hydraulic cam could have the same duration numbers listed at .050, but the solid cam would actually deliver slightly less effective duration because of the lash ramp and would need to be increased slightly to deliver the same duration at the valve.

The shape of a flat tappet cam versus a roller can be misleading as well. The wheel of a roller cam interacts in a 1:1 relationship to the cam lobe following it exactly while the cam lobe slides across the entire surface of a flat tappet lifter. Essentially, a roller cam requires a physically larger lobe to create the same valve opening and closing events as a similarly spec’d flat tappet. This fundamental difference favors the roller cam as rpm and spring pressure increases.

“Solid cams use a lifter than has no such internal adjustment, which gives them an extremely precise transfer of the cam profile to the valvetrain.”